Captive marmosets that listened in on recorded vocal interactions between other monkeys appeared to understand what they overheard - and formed judgements about one of the interlocutors as a result, according to behavioral analyses and thermal measurements that corresponded with the marmosets' emotional states. The findings suggest that the eavesdropping monkeys perceived these vocalizations as "conversations" rather than isolated elements and indicate that, on the whole, they prefer to interact with cooperative rather than noncooperative individuals. However, the researchers observed notable differences in how male or female and breeder or helper animals (those without their own offspring) reacted after eavesdropping. While behavioral studies have increasingly offered glimpses into the social lives of primates, they tend to lack reliable measurements of the observer's "inside perspective" - the inner workings of her or his mind. To delve into the minds of snooping marmosets, Rahel Brügger and colleagues presented 21 captive-born adult marmosets with recordings of an opposite sex adult making either food-offering calls or aggressive chatter calls in response to begging infants. As controls, the researchers also played back each call individually. They then assessed changes in the marmosets' nasal temperatures, which correspond with changes in the autonomous nervous system that accompany shifting emotional states. Since the marmosets showed different subtle temperature changes in response to the holistic encounters than to their individual components, Brügger et al. concluded that the monkeys perceived them as conversations. Female and male helpers displayed the strongest emotional responses, but in different ways. Female helpers showed steep temperature drops (indicating strong emotional reactions) after listening to both positive and negative interactions between a male adult and a baby, while male helpers showed temperature increases after listening to negative interactions or to an individual playback of an adult female. Next, the researchers tested whether the monkeys preferred the cooperative, food-sharing individuals or the noncooperative ones by opening both testing compartment doors from which the recordings had been played. Overall, marmosets were less likely to enter the compartment from which they had heard the negative interaction, suggesting that they tend to prefer cooperative individuals. However, the authors note that the behavioral expression of this preference is very context-specific, with marmosets appearing to weigh social costs before acting.
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Science Advances